Calibration of rotary viscometers must be checked periodically to ensure that the instrument measurement system is not damaged or out of calibration due to changes or wear in the measurement mechanism. Rotary viscometers, for example, use a torsion spring to measure the torque produced by the rotational motion of a spindle in test fluid. The shear rate of the fluid is proportional to the viscosity, and determined by the spindle geometry and relative speed of rotation between the spindle and the test fluid. The torque to rotate the spindle is proportional to the viscosity of the fluid.
Normally, instrument calibration is checked with standard fluids. These standard fluids are test samples with a documented viscosity. Therefore, these standard fluids produce a known torque on the torsion spring at a controlled shear rate and temperature. This calibration method is useful, but prone to errors that can impact the accuracy of the calibration under real-world conditions, especially if the standard fluids are not kept at a precise temperature and properly maintained.